1. Field of the Invention
The present invention is directed to bonding cyanate ester composites to one another, and more particularly, to an adhesive that bonds cyanate ester composites while also being plateable with strong metal adhesion upon treatment by a surface etching process.
2. Description of Related Art
Metals are commonly employed in manufacturing because they offer high degrees of ductility and strength as well as high conductivity. However, metals are generally heavier than other common materials like plastics and non-metallic composites, such that the positive characteristics attributable to metals often come at the cost of increased product weight. Increased product weight is particularly a concern in industries manufacturing vehicles of transport such as automobiles, aircraft, and spacecraft, as well as payloads of such vehicles, given that increased weight adversely affects fuel economy. Moreover, metals typically have a higher coefficient of thermal expansion than non-metallic composites, such that metals exhibit more dimensional change given temperature differentials.
Accordingly, these industries have increasingly incorporated non-metallic, lighter-weight materials such as plastics into automobiles and aircraft in an effort to economize fuel and maintain dimensional stability. However, plastics are not universally suitable as substitutes for metals. For example, while plastics offer high degrees of ductility and strength, plastics are relatively nonconductive materials. Thus, plastics cannot supplant metals used as electrical, thermal, or microwave conductors.
It is therefore desirable to plate a metal coating onto plastic, thereby simultaneously realizing the benefits of both metals and plastics. More specifically, metal plating on plastic materials allows the use of these lighter-weight plastic materials for the bulk of components and minimizes the amount of metal required to achieve a highly conductive surface. Of particular interest is metal plating on cyanate ester polymer composites of cyanate ester resin and graphite fiber, since such composites have certain unique advantages that make them very useful for specific applications, such as applications in communications spacecraft. Specifically, cyanate ester polymer composites can be formulated in ways that make them very resistant to even minute dimensional changes that would otherwise occur as a result of temperature changes or the absorption and desorption of moisture in the presence of air.
Two methods have been developed by which a cyanate ester composite surface may be treated in preparation for the subsequent plating of metal, both of which are the subjects of previously-filed applications assigned to the same assignee as the present application. An application entitled "Preparation of Cured Cyanate Ester Resins and Composites for Metal Plating" (Ser. No. 08/339,390, filed Nov. 14, 1994 U.S. Pat. No. 5,569,493), recites treating the surface of cyanate esters polymers and composites with a preheated solution of an alkali metal salt of an alkoxide to achieve greater adhesion between the surface and subsequently-plated metals. A second application filed on even date therewith and entitled "Preparation of Cyanate Ester Polymers and Composites for Metal Plating" (Ser. No. 08/339,380) recites treating the surface with a preheated solution comprising a quaternary ammonium hydroxide or a primary amine.
The above-described surface treatment methods achieve greater adhesion for cyanate ester polymer composites by chemically etching the composite surfaces. Chemical etching texturizes the composite surfaces, thereby providing mechanical anchoring sites for the plating of metal such that the metal adheres to the composite. Scanning electron micrographs have revealed that surface texturing derives from microcracking in the cyanate ester resin that is wedged between the stiff graphite fibers of the composite.
Adhesion between cyanate ester composites and subsequently-plated metals is further improved by practicing the invention described in an application entitled "Cyanate Ester Films that Promote Plating Adhesion to Cyanate Ester Graphite Composites" (Ser. No. 08/507,178, filed on Jul. 26, 1995, now abandoned). That application recites improving the adhesion between cyanate ester composites and a subsequently-plated metal by applying a film of cyanate ester resin to the composite surface prior to surface etching. The cyanate ester resin film provides a homogenous surface upon which to obtain uniformity of adhesion between the cyanate ester resin composite and a subsequently-plated metal. In the absence of a cyanate ester resin layer, the inconsistent distribution of graphite (or other composite-forming material) across the surface of the cyanate ester resin composite leads to variability in adhesiveness with a subsequently-plated metal.
While the above-described surface treatment methods provide the necessary surface texturing for subsequently plating a metal onto the treated composite surface, the structural adhesives presently available to bond cyanate ester composite articles together are not satisfactorily plated with metal upon completion of these same surface treatment methods. Articles made of cyanate ester composites are often pieced together to form complicated structures, since it would be impractical both technically and economically to fabricate complicated structures as a single piece. Since presently-available structural adhesives fail to satisfactorily plate by the same processes used to prepare their cyanate ester composite adherends, structures assembled from cyanate ester composite articles are not comprehensively plated with metal. More specifically, there are delineations in the metal plating at the exposed bondlines. Discontinuities in the plating of composite structures preclude their use in certain sensitive applications, such as satellite microwave circuitry.
Presently, structural adhesives available to bond cyanate ester composite articles together include cyanate ester adhesives and epoxy-based adhesives. While certain commercially-available cyanate ester adhesives may be successfully prepared for metal plating by the above-described surface etching processes, these adhesives require cure temperatures exceeding 120.degree. C., at which articles made of graphite fiber reinforced cyanate ester composites warp. In contrast, while certain commercially-available epoxy-based adhesives cure at room temperature to bond cyanate ester composites without warpage, these adhesives either plate poorly following the above-described surface etching treatments or, upon plating, exhibit unacceptably low lap shear and peel strengths. For example, aluminum-filled epoxy adhesives (such as Ciba Geigy's RP4036 adhesive including RP1500 hardener) fail to plate properly following treatment by the above-described etching processes without an additional special activation step. Commercially-available silver-filled epoxy adhesives contain large amounts of silver filler to achieve electrical conductivity; as a result, metal subsequently plated onto these adhesives (following surface etching) suffers from poor lap shear and peel strengths because of the large amount of filler material.
Thus, a need remains for an adhesive that develops strong bond joints to cyanate ester composites while also being capable of developing strong adhesion to a subsequently-plated metal given surface texturing treatments that are performed simultaneously on the adhesive and its composite adherends. The adhesive must be easy to apply and readily curable at temperatures low enough such that the adherend cyanate ester composite parts are not warped. Finally, the adhesion achieved between the exposed bondline of the adhesive and the subsequently-plated metal must be uniform and reproducible, since a single plating failure in an application such as a satellite microwave circuit can be economically catastrophic. In particular, in order to be suitable for the above-described demanding applications, the plating adhesion value should be 5 as measured per ASTM D3359 simultaneously with the plating to a cyanate ester composite substrate, the distribution of peel strength should have a mean, or average, value that is greater than two standard deviations above 1.5 lbs/inch-width, and the distribution of lap shear should have a mean value that is greater than two standard deviations above 2,000 psi.